Efficient H 2 O 2 Production via H 2 O Oxidation on an Anode Modified with Sb‐Containing Mixed Metal Oxides

Anodic H2O2 Generation in Carbonate-Based Electrolytes-Mechanistic Insight from Scanning Electrochemical Microscopy

For the anodic H2O2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H2O2 generation. Previous efforts made on composition optimization found that the impact of the molar fraction of carbonate species varies for different anodes, and therefore, controversies remain concerning the reaction pathways as well as the species involved in H2O2 formation. Considering that water oxidation results in the liberation of protons within the anode microenvironment, the corresponding acidification would cause an equilibrium shift between carbonate species, which in turn may modulate the reaction pathway. We determined the changes in the fraction of carbonate species in the vicinity of an anode by performing local pH measurements using a Au nanoelectrode positioned in close proximity to an operating anode by shear-force scanning electrochemical microscopy (SECM). It could be confirmed that the main anionic species at the interface is HCO3 -, at potentials where H2O2 is preferentially formed, regardless of the pH value in the bulk. The simultaneous use of a Au-Pt double barrel microelectrode in generator-collector SECM measurements demonstrates that the local HCO3 - concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte.

Peroxodicarbonate - a renaissance of an electrochemically generated green oxidizer

The direct anodic conversion of alkali carbonates in aqueous media provides access to peroxodicarbonate, which is a safe to use and green oxidizer. Although first reports date back around 150 years, its low concentrations and limited thermal stability have consigned this reagent to oblivion. Boron-doped diamond anodes, novel electrolyser concepts for heat dissipation, and the mixed cation trick allow record breaking peroxodicarbonate concentrations >900 mM. The electrochemical generation of peroxodicarbonate was already demonstrated on a pilot scale. The inherent safety is ensured by the limited stability of the peroxodicarbonate solution, which decomposes under ambient conditions to oxygen and facilitates subsequent downstream processing. This peroxide has, in particular at higher concentrations, an unusual reactivity and seems to be an ideal reagent when peroxo-equivalents in combination with alkaline base are required. The conversions with peroxodicarbonate include the Dakin reaction, epoxidation, oxidation of amines (aliphatic and aromatic) and sulfur compounds, deborolative hydroxylation reactions, and many more. Since the base equivalents also represent the makeup chemical for pulping plants, peroxodicarbonate is an ideal reagent for the selective degradation of lignin to vanillin. Moreover, peroxodicarbonate can be used as a halogen-free bleaching agent. The emerging electrogeneration and use of this green platform oxidizer are surveyed for the first time.

Super-Oxidizing Covalent Triazine Framework Electrocatalyst for Two-Electron Water Oxidation to H2 O2

Electrochemical two-electron water oxidation (2e WOR) is gaining surging research traction for sustainable hydrogen peroxide production. However, the strong oxidizing environment and thermodynamically competitive side-reaction (4e WOR) posit as thresholds for the 2e WOR. We herein report a custom-crafted covalent triazine network possessing strong oxidizing properties as a proof-of-concept metal-free functional organic network electrocatalyst for catalyzing 2e WOR. As the first-of-its-kind, the material shows a maximum of 89.9 % Faradaic Efficiency and 1428 μmol/h/cm2 H2 O2 production rate at 3.0 V bias potential (vs reversible hydrogen electrode, RHE), which are either better or comparable to the state-of-the-art electrocatalysts. We have experimentally confirmed a stepwise 2e WOR mechanism which was further computationally endorsed by density functional theory studies.

Photocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water

Hydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.

Electrochemical generation of hydrogen peroxide from a zinc gallium oxide anode with dual active sites

Electrochemical water oxidation enables the conversion of H₂O to H₂O₂. It holds distinct advantages to the O₂ reduction reaction, which is restricted by the inefficient mass transfer and limited solubility of O₂ in aqueous media. Nonetheless, most reported anodes suffer from high overpotentials (usually >1000 mV) and low selectivity. Electrolysis at high overpotentials often causes serious decomposition of peroxides and leads to declined selectivity. Herein, we report a ZnGa₂O₄ anode with dual active sites to improve the selectivity and resist the decomposition of peroxides. Its faradaic efficiency reaches 82% at 2.3 V versus RHE for H₂O₂ generation through both direct (via OH^-) and indirect (via HCO₃^-) pathways. The percarbonate is the critical species generated through the conversion of bicarbonate at Ga-Ga dual sites. The peroxy bond is stable on the surface of the ZnGa₂O₄ anode, significantly improving faradaic efficiency.

Discovery of LaAlO3 as an efficient catalyst for two-electron water electrolysis towards hydrogen peroxide

Electrochemical two-electron water oxidation reaction (2e-WOR) has drawn significant attention as a promising process to achieve the continuous on-site production of hydrogen peroxide (H₂O₂). However, compared to the cathodic H₂O₂ generation, the anodic 2e-WOR is more challenging to establish catalysts due to the severe oxidizing environment. In this study, we combine density functional theory (DFT) calculations with experiments to discover a stable and efficient perovskite catalyst for the anodic 2e-WOR. Our theoretical screening efforts identify LaAlO₃ perovskite as a stable, active, and selective candidate for catalyzing 2e-WOR. Our experimental results verify that LaAlO₃ achieves an overpotential of 510 mV at 10 mA cm^-2 in 4 M K₂CO₃/KHCO₃, lower than those of many reported metal oxide catalysts. In addition, LaAlO₃ maintains a stable H₂O₂ Faradaic efficiency with only a 3% decrease after 3 h at 2.7 V vs. RHE. This computation-experiment synergistic approach introduces another effective direction to discover promising catalysts for the harsh anodic 2e-WOR towards H₂O₂.

Suppressing Water Dissociation via Control of Intrinsic Oxygen Defects for Awakening Solar H2 O-to-H2 O2 Generation

BiVO₄ theoretically has a thermodynamic activity trend toward highly selective water oxidative H₂ O₂ formation, but it is more inclined to generate O₂ in practical. The influence of intrinsic oxygen vacancy (O_vac ), especially, on surface reactivity, has never been considered as a possible activity loss mechanism in the synthetic BiVO₄ . In this work, it is theoretically and experimentally demonstrated that the intrinsic surface O_vac is responsible for lower H₂ O₂ evolution activity via promoting water dissociation to form intermediate. Through an annealing process under a V₂ O₅ rich atmosphere, the surface O_vac can be eliminated that awakens the photoelectrochemical (PEC) water oxidative H₂ O₂ activity in a NaHCO₃ electrolyte, which achieves an average of 58.4%, and increases by up to 4.28 times of the one annealed in air. This work offers a general understanding of catalytic activity loss and may be extended to other photo or electrocatalysts for catalytic selectivity regulation.

Synthesis and characterization of Sb2O3: a stable electrocatalyst for efficient H2O2 production and accumulation and effective degradation of dyes

Sb₂O₃ films are synthesized and characterized as electrocatalysts showing efficient H₂O₂ production and accumulation properties.